Well, we've found Martian meteorites on Earth, it's almost a certainty that some Earth rocks made it to Mars.
And I thing about the first half dozen to dozen probes that we sent there didn't have extensive bacterial decontamination before they were launched, that's a relatively recent protocol.
We should terraform it. Send up some lichens, molds, mosquitoes and dumpster divers.
Before you know it, somebody would be selling condos.
What they need to do is the "nonenvironmental" thing. Ram a few comets and asteroids into it. Give it a little more gravity, a little more atmosphere, and a little more liquid.
I would start with Phobos and Diemos.